1. Field of the Invention
The concentration of DO in water has long been a significant concern in water quality management and certain minimum levels thereof are necessary for aquatic biological systems and for either separate treatment of or concurrent assimilation of treated municipal and industrial wastewaters. In short, DO is an important measure of the general quality of surface waters.
With respect to the first principal embodiment of the instant invention as it relates to a new and improved passive aeration device for tailwater management, it is well recognized that all impoundments affect DO. DO reductions in some reservoirs may not be severe enough to affect aquatic life, while in others, significant DO reductions seasonally lead to releases low in DO. In still other reservoirs, reductions intermittently lead to releases low in DO.
Many factors affect DO. Identification of these for each reservoir permits cost-effective mitigative measures. Recognition of DO patterns can help in assessing potential for other undesirable water quality that may result from low DO. For example, hydrogen sulfide sometimes associated with very low DO concentrations can be toxic to aquatic life. Iron and manganese sometimes associated with low DO can consume DO added by aeration systems, contribute to slime growths in turbine systems, and cause staining in water supplies.
It will be appreciated that DO concentrations at any time or location are the result of a delicate balance of oxygen-producing and oxygen-consuming processes. Contact with the atmosphere and photosynthesis by aquatic plants are natural sources of oxygen. The atmosphere is generally the most significant source. In conjunction with water temperature, atmospheric pressure determines the saturation concentration of DO in fresh water. Whenever DO is less than the saturation level, oxygen is absorbed from the atmosphere. Although the atmosphere usually serves as a source of oxygen, it also serves as a "sink" when DO is supersaturated.
Oxygen-consuming processes include 1) decomposition of dissolved, suspended, and settled organic matter from municipal and industrial wastewaters, natural and manmade nonpoint sources, and dead aquatic plants; 2) aquatic plant respiration; and 3) nitrification (oxidation of ammonia and nitrite to nitrates).
The balance between natural production and consumption of DO depends on temperature (affects the saturation DO level and rates of various biological processes); contact between water and the atmosphere (depends on hydraulic characteristics such as turbulent mixing and density currents); morphology and substrate characteristics; nutrient availability for aquatic plants; meteorology (especially wind speed, temperature, pressure, and solar radiation); and toxicants (which can suppress microorganisms involved in consumption processes and photosynthesis).
The effects of various reservoir characteristics on DO can be described generally, and DO levels that can be expected in releases from different types of projects can be estimated. However, the only way to quantify DO levels at specific projects is to use site-specific data, often employing a mathematical water quality model.
For a more detailed and definitive treatment of factors affecting water quality in the releases from hydropower reservoirs including 1) the effects of DO on thermal stratification in storage, transitional, and mainstream reservoirs, 2) the effects of environmental processes by both longitudinal zones and thermal layers, and 3) the effects of sulfide, iron, and manganese constituents in certain reservoir waters see Ruane, Richard J., and Hauser, Gary E., "Factors Affecting Water Quality in the Releases from Hydropower Reservoirs," Proceedings of the 52nd American Power Conference, Illinois Institute of Technology, Chicago, 1990. In addition, more specific details of developments and measures which have been implemented by the assignee of the instant invention to increase minimum flows below selected hydropower projects and to improve DO contents of releases therefrom can be found in "Improving Reservoir Releases," Tennessee Valley Authority, TVA/ONRED/AWR 87/33, Knoxville, Tenn., 1987. For purposes of teaching, disclosing, and claiming the instant invention, the teachings and disclosure of both references, supra, i.e. Ruane and TVA, respectively, are hereby incorporated herein by reference thereto.
Although a major thrust of the instant invention is directed to enhancing water quality downstream of hydroprojects by increasing the DO content thereof, this particular embodiment necessarily must also be concerned with and directed to providing requisite minimum flows in river/tributary streams. Accordingly, the remainder of this section is intended as an abbreviated overview of some of the methods investigated and/or utilized for providing such minimum flows.
Turbine Pulsing: Turbine pulsing refers to operation of an existing turbine at a hydropower dam for a short duration at regular intervals. The objective of such pulsing scheme is to maintain a minimum flow higher than hydroproject leakage whenever the turbines would not otherwise be generating, such as at night, on weekends, during filling of the reservoirs, or during flood protection operation for downstream reservoirs. In the immediate tailrace, turbine pulsing causes flow fluctuations between a minimum that represents leakage rate and a maximum of one turbine discharge. A mile or so downstream, the minimum flow observed is greater than the leakage value and the maximum is substantially less than the one turbine value. Farther downstream, the fluctuations continue to dampen so that the minimum and maximum flows approach the temporal mean flow.
Unsteady flow modeling studies of several tailwaters have shown that pulsing with existing turbines can provide an essentially constant flow within a few miles downstream of many projects using a proper combination of pulse flow, duration, and interval. The pulsing flow used is typically the most efficient discharge for the smallest turbine at the project; the pulse duration typically ranges from 15 minutes to 1 hour; and the interval between pulse starts is typically 2 to 6 hours for a tailwater with no reregulation weir, and 8 to 24 hours for a tailwater with a reregulation weir.
Reregulation Weirs: The main objective of a reregulation weir is to sustain an increase in minimum flow between generating periods by discharging stored water slowly after periodic refill from the upstream dam. The pools behind these weirs normally have enough storage to maintain target minimum flows for 8 to 12 hours before they need to be refilled by releases from the upstream dam. The weirs are overtopped quickly during generation and do not provide any significant reduction of peak discharge.
Reregulation weirs are low head dams usually less than about 12 feet high with low level releases (usually submerged pipes) located in the first few miles downstream from a major hydroproject. Weirs may be constructed of concrete but use of porous media with an impermeable liner has decided benefits for reducing dangerous flow conditions at the weir. Releases from reregulation weirs are by overtopping and open pipe flow (during generation) and by valve and pipe flow (off generation). Weir height and location are designed to maximize the length of tailwater where minimum flow is sustained (i.e., downstream of the weir); provide adequate storage volume to sustain a minimum flow and minimize refill frequency; and minimize the increase in backwater on the upstream turbine.
Float-actuated valves in low level pipes through the weirs have been shown to provide fairly constant flow as the weir pool drains. Such arrangements provide drain time durations of from 2 to 3 times that possible with unvalved pipes. Minimum flows are maintained at a fairly constant rate (up to 20% deviation from target minimum flow rate) below the weir except for increase in flow due to overtopping during the refill cycle. It is normal practice to surcharge the weir pool and overtop the weir with each pulse to maximize the water stored in the channel above the weir. This can add as much as an hour to flat pool drain times.
A power loss cost is associated with use of weirs due to the need to refill when turbines would not otherwise be used, such as off-peak hours, weekends, and during spring filling of the reservoirs. Weirs can reduce the required frequency of pulsing on tailwaters where the design objectives described previously can be achieved.
Small Turbine Addition: A small turbine can be sized to provide the target flow rate continuously between periods of normal hydrogeneration. The small unit discharge is normally only a small fraction of the capacity of larger units designed for the sole purpose of hydrogeneration. Small units are usually not as efficient as the large units, and are usually run constantly. Thus, even though power is being generated with the small unit, a power loss cost is still incurred due to reduced efficiency and off-peak use. Small units are not always technically feasible because they require a convenient access to impounded water and an outlet designed to pass the target minimum flow. Costs vary from site to site, but small unit additions tend to be more expensive than turbine pulsing and weir options.
Sluicing/Spilling: Release of water through sluice gates or over gated spillways is sometimes used to provide emergency flows downstream. However, sluicing and spilling have such high power loss costs that they are not usually considered for routine minimum flow maintenance. Like the small unit, these methods can provide more or less constant flows. However, existing systems are not normally designed for low discharges, so it is often difficult to regulate the flows down to target levels without outlet modifications. Operational problems such as cavitation may also develop.
2. Description of the Prior Art
Historically, weirs have been built to regulate water supplies. Water flowing over a weir drags air in its wake as it plunges into the pool downstream therefrom. Gameson reported weirs are good aerators. (Gameson, A. L. H., "Weirs and the Aeration of Rivers", J. Inst. of Water Engineers, 11:477-490, 1957.) Water falling over weirs can gain approximately 1 mg/L of DO for every foot of fall height. With interest in aerating weirs on the rise, major contributions subsequently were made by Avery, Sean and Novak, Pavel, "Oxygen Transfer at Hydraulic Structures," J. of the Hydraulics Division, ASCE, 104 (HY11):1521-1540 (Nov. 1978); Markovsky, M. and Kobus H., "Unified Presentation of Weir Aeration Data", J. of the Hydraulics Division, ASCE, 104 (HY4):562-568, April 1978; and, Nakasone, H., "Study of Aeration of Weirs and Cascades", J. of Env. Eng., 113 (1):64-81 (Feb. 1987), among others.
The simplest weir design is a straight weir normal to flow in the river channel. Unfortunately, with this simple design aeration performance and safety are decreased as flow rate thereover increases. Previous studies have determined that optimal aeration occurs at 0.7 cfs/ft (Nakasone, supra) and unsafe hydraulic conditions occur at flow rates higher than 1.5 to 2.0 cfs/ft with 3 to 4 feet drop heights into deep plunge pools [Leutheusser, H. J., and Brink, W. M., "Downproofing of Low Overflow Structures," J. Hydr. Eng., Vol. 117, No. 2, pp. 205-213 (Feb. 1991)]; [Hauser, G., "Full Scale Physical Modeling of Plunge Pool Hydraulics Downstream of a Vertical Weir", TVA Report WR28-1-590-153 (May 1991)]. These safety considerations have been instrumental in the decision process which led the assignee of the present invention to build a labyrinth aerating weir at its South Holston tailwater, since such type of weir can be designed to effect low specific discharge rates. Although aeration performance can be improved dramatically, and dangerous hydraulic jumps eliminated, the installation of a labyrinth weir represents and requires considerable initial capital investment. Accordingly, in situations wherein only the regulation of water supply and maintenance of minimum flow is the primary consideration such a relatively expensive approach is not required. Thus both types of weir design (straight and labyrinth) may have important characteristics for consideration depending on the design requirements for a given situs.
With these factors in mind, the assignee of the present invention developed its own functional design for its so-called timber crib weir which has been determined to be safe and effective for improved minimum flow below hydropower dams between generating periods thereof. Another distinct functional design developed and tested by said assignee has been one of the labyrinth weir type for enhancing both minimum flow and aeration. One driving force for investigating the design, costs, installation and operating characteristics of the labyrinth type aerating weir has been the results of a fairly recent analyses of the relationship between benthic communities and DO at well over a dozen tailwaters over the Tennessee Valley which analyses demonstrated that there was a statistically significant, direct correlation between measures of benthic community health and DO concentrations in such tailwaters. Although improvements were fairly linear between substantially no measurable DO and about 7 mg/L of DO in water tested, there was shown to be a dramatic shift from benthic communities dominated by organisms tolerant of low DO to more diverse communities, including those intolerant of low DO at a DO level of about 4 mg/L.
With both the timber crib weir and the labyrinth type weir design, supra, a target minimum flow is sustained by slow drainage of the weir pool between periodic refills. With the labyrinth weir, aeration occurs during generation via overtopping. Both weirs are designed to maximize the value of the tailwater while minimizing backwater on the upstream turbine, unsafe hydraulic conditions, and environmental disturbance.
Design steps common to both weirs included selecting a target minimum flow, determining weir height and location, and configuring low-level pipes and valves for flow control.
Target minimum flows were selected in a tradeoff evaluation that considers 1) visual observation of flow tests; 2) modeled incremental physical changes with increased flow; 3) professional judgment of aquatic benefits; and 4) assessment of impacts to recreation, upstream reservoir pools, and annual power production. Resulting minimum flow targets ranged from 50% to 100% of the unregulated 7-day 10-year low flow. Physical changes resulting from such minimum flows vary widely across tailwaters. Depth increases of several tenths of a foot and wetted area increases of 25% to 65% in riffles are common, relative to leakage conditions.
The weir impoundment must have enough storage capacity to achieve desired drain times with a reasonable weir height. Ideally, a weir pool should require only one refill per day (24 hour drain time) so that a single refill pulse can be scheduled during peak power demands. However, in the Tennessee Valley River basin this would require weirs 10 to 15 feet high, thereby creating concerns about upstream flooding. Weir heights in the more typical 5 to 10 feet range require two or three refills per day (8 to 12 hours drain time) forcing at least one off-peak refill. For weirs with 12 to 24 hours of drain time, two refill pulses can sometimes be placed on the "shoulders" of the peak demand period, reducing off-peak costs. To maximize channel storage upstream of the weir, the weir pool can be surcharged to slightly overtop the weir at each refill and add as much as an hour to flat pool drain times. Weirs should be located near the upstream dam to maximize the length of improved tailwater (downstream of the weir), but they must not add to backwater on the turbines. A location within the first 3 miles below the powerhouse is normal. Also to be avoided are sites downstream from tributaries with poor water quality and sites requiring major dewatering during construction. Land for the weir, its maximum pool, and construction staging areas must be acquired.
Other design considerations included low-level pipes and float-actuated butterfly valves [Loiseau, et al., "Modeling and Verification of the Clinch River Weir," TVA Engineering Laboratory, Norris, Tenn. (1983)], which can be installed to maintain and prolong the minimum flow between generating periods. The butterfly valves, connected by hinged arms to a Styrofoam float, act in conjunction with unvalved pipes to provide constant flow by opening gradually as the weir pool drops. Both 12-inch and 18-inch diameter pipes have been used. Use of few large diameter pipes rather than numerous small diameter ones can reduce costs in material, float and valve manufacture, installation, and maintenance. Pipes should be oversized or extra pipes added and capped to provide reserve capacity. Oversized segments can be installed in the weir with smaller segments attached upstream. Accordingly, the diameter of the attached pipes can be changed, and maximum discharge capacity is limited only by the large pipe segments.
As noted above, when the primary design consideration is only for minimum flow, the preferred design is a timber crib weir with pipes and valves as described above. The timber crib is rock-filled with heavy steel grating over the fill. An impermeable membrane at the upstream weir face prevents throughflow.
Improper weir design will produce a downstream hydraulic jump or "keeper" that can trap small boats or swimmers. These conditions are avoided by building a wide (upstream to downstream), porous structure which steps or slopes down gradually to the downstream face. Some flow moves through the weir and exits near the stream bed, dissipating energy along the overflow path, rather than all at once in a turbulent hydraulic jump. This design has evolved from physical modeling and prototype testing. Flow conditions are sensitive to parameters such as tailwater elevation. Thus, site-specific physical modeling and shaping is recommended [Loiseau, et al., "Modeling and Verification of the Clinch River Weir," TVA Engineering Laboratory, Norris, Tenn. (1983)].
The Norris crib weir has successfully maintained minimum flows (200 cfs for 12 hours between refills) since 1984 without major impact on Norris Dam operations. Trout fishing activities in the tailwater accelerated after weir completion, and in September 1988, the state record brown trout (28 lbs, 12 oz) was caught just below this weir.
As noted above, when the weir design considerations must address both minimum flow and aeration, the prior art preferred design is a labyrinth weir. This weir design has an extended crest length to reduce unit discharge during overtopping. In this manner, this design decreases the intensity of the plunge pool roller and improves aeration. Aerating weir design considerations focus on proper crest length and labyrinth shape. The crest length selected must result in a unit discharge low enough to produce safe plunge pool conditions and still result in effective aeration. Unit discharges of 7 cfs/ft over low head weirs have resulted in fatal rollers, while unit discharges of 0.5 cfs/ft create thin sheets of water with trivial downstream recirculation. A range of unit discharges from 0.5 to 3.5 cfs/ft was tested full scale during design of the South Holston labyrinth weir (Hauser, 1991). Plunge pool hydraulics below a vertical weir segment were measured and observed by walking and swimming in the recirculation zones with safety gear. Results suggested 2 cfs/ft as a threshold above which flow conditions become mildly troublesome to free swimmers. The South Holston weir (2100 ft overflow length) was designed to operate at 1.2 cfs/ft during normal turbine operation and about 2 cfs/ft during flood operation (less than 1% of the time). Plunge pool depths will average 3 feet with a maximum of 4.5 feet to allow an adult person to walk instead of swim for self-rescue or rescue of small children.
Aeration is achieved primarily by bubbles entrained as the overflow nappe impinges on the plunge pool. Aeration efficiency is affected by drop height, unit discharge, plunge pool depth, and oxygen deficit (Nakasone, supra). Additional aeration occurs down the length of the labyrinth legs due to reentrainment of bubbles along the flow path. Based on preliminary testing, the South Holston weir should increase DO by over 4 mg/L when upstream DO is 3 mg/L and unit discharge is 1.2 cfs/ft, with a 4.5 feet fall height (headwater to tailwater).
The labyrinth shape must be planned after the crest length is determined. The labyrinth must fit reasonably within the river channel, provide good bubble zones, and achieve uniform overflow conditions all along the weir crest. Weir segments normal to river flow are designed to be non-overflow to avoid nappe convergence in corners which might increase recirculation intensity. To avoid long labyrinth legs, numerous bays are desirable. However, the number of bays is limited by the channel width and the bay width needed for full development of bubble zones along the sides of each labyrinth bay. Non-overflow segments should be minimized to reduce costs. Crest length and these shape constraints determine bay length and leg angle. Water surface profiles in headwater bays need to be computed to check for nonuniform overflow conditions which might produce dangerous hydraulics along the weir.
Specifications for the South Holston labyrinth weir call for concrete and wood with weir walls being pressure-treated, tongue-and-groove wood members placed horizontally atop lower members and supported by reinforced concrete piers anchored into bedrock. Piers are designed to be slotted vertically to receive wall members in a stoplog arrangement. Intermediate concrete buttresses at midpoints between the slotted piers are included to support lower wood members. A concrete leveling pad between each slotted pier follows the channel bottom beneath the weir wall. Slotted piers include Tee (T) pipes to ventilate behind the nappe. Pipes and valves like the timber crib weir are included for minimum flow control.
Advantages of both weir designs, supra, are evidenced in hydraulic and aeration performance, weir safety, and ease of construction. The pipe and valve assembly is a proven low maintenance way to regulate flow. The porous timber crib eliminates dangerous hydraulics, and is easily constructed with inexpensive materials. Overflow conditions are navigable without danger. Disadvantages of weirs like the timber crib are that it is attractive to the public as a fishing pier in a location of rapidly varying water levels, in spite of efforts to prohibit such use and, of course, it is useful only for effecting minimum flow. However, if this is the suggested design criteria, then cost-effectiveness of a timber crib weir for maintenance of said minimum flow rivals that of turbine pulsing and usually exceeds that of small turbine additions or turbine bypass options.
The labyrinth weir design achieves both minimum flow and aeration objectives within a single structure and therefore is considerably more versatile than the crib weir, supra. With this design both aeration and safety increase as the crest is lengthened. Aeration is passive, so all releases from the upstream dam are aerated, regardless of origin. Aeration is downstream of the hydroplant, avoiding efficiency losses and cavitation problems of in-plant, turbine intrusive methods. Extended crests efficiently pass flows with less inundation and head loss on the upstream turbine than with straight weirs. The labyrinth weir cannot be used as a fishing pier and, accordingly does not present a structure having this undesirable public attraction characteristic. Disadvantages of the labyrinth weir are that it is non-navigable, it requires excessive crest length to safely aerate high flow applications, it reduces fish passage, and although aerating weir costs are equivalent to certain combinations of other methods for minimum flow and DO improvement, this design requires considerable capital investment due to the high costs associated with construction.
It should now be appreciated that the prior art relating to the design of weirs, for providing both control of minimum tailwater flow and aeration thereof required a new initiative wherein the restrictive disadvantages heretofore encountered can be simply and inexpensively overcome. This need for substantial improvement in this area is further appreciated when other alternatives for maintaining minimum flow and providing requisite aeration in tailwaters are considered, including headwater reservoir modifications with diffusers (forebay, trashracks, or penstocks), or active destratification means (forced air or water circulation) or a plethora of retrofit turbine/draft tube modifications (hub baffles, turbine runners, draft tube rings, and head cover injectors).